The ice cap present in
Antarctica has an average thickness of approximately 2000 metres, and an being
laid down as snow and compressed over hundreds of thousands of years. The layer
of snow at the surface is porous and contains bubbles of air, with the makeup of
the atmosphere at the time of deposition. The air above Antarctica is free from
local pollutants and contains a true representation of the makeup of the
atmosphere.

Deep within the ice
there are bubbles of air present that have been isolated since the ice was
compressed. The ice is extracted by drilling into the ice cap and removing cores
of ice. The resulting core of ice, consisting of many sections totalling several
hundred meters in length, represents a timeline containing samples of the
atmosphere at the time the snow was laid down. The cores are preserved by
keeping them frozen before analysing the contents.

The concentrations of
the gases in the bubbles are analysed by grinding the ice at very low
temperatures and condensing the gases released. The concentration of CO2
present gives a measure of the extent of global warming. Dust in the ice can be
linked to periods of cold dry weather with an increase in desert areas and wind
speed.

The figure below shows
the trend over thousands of years of CO2, temperature and parts per
million of dust obtained by the study of ice cores from Antarctica. An increase
in the level of CO2 is linked to an increase in the temperature while
during periods of lower temperature, dust levels are higher.